As a handheld-type, that is, portable-type engine power tool with using an engine as a driving source, there are a chain saw, a power cutter, and others. In the chain saw, a saw chain which is a chain-shaped saw blade for cutting a workpiece such as wood is provided in a power tool main body. In the power cutter, a disc cutter for cutting or creasing stone or concrete is provided in a power tool main body. An engine mounted on such an engine power tool includes an engine main body including: a crank case to which a crank shaft is attached so as to be rotatable; and a cylinder in which a piston is embedded so as to be linearly reciprocatable, and a cooling fan is attached to one protruding end of the crank shaft, so that it is a forced cooling engine. Cooling air generated by the cooling fan is blown along the engine toward a side of the engine opposite to the cooling fan, so that the engine is forcibly cooled. When the engine stops, the rotation of the cooling fan stops, and therefore, the engine is cooled by natural cooling after the engine stops.
In order to supply an air-fuel mixture of fuel and air to the engine, a carburetor is attached to the engine so as to interpose an insulator. When the engine stops, the flowing of the air-fuel mixture inside the carburetor and the insulator also stops. Therefore, a cooling effect of the carburetor and the insulator by the air-fuel mixture is lost, and therefore, heat conduction occurs from the cylinder to the carburetor through the insulator so as to increase a temperature of the carburetor. If the increase in the temperature of the carburetor is too large, most of gasoline (petrol), that is, fuel, is vaporized, and therefore, a sufficient amount of the fuel cannot be supplied from the carburetor to the engine, which results in difficulty in restart of the engine. More particularly, downsizing is strongly demanded for the engine used for the portable engine power tool, and therefore, it is required to make the insulator as short as possible. Therefore, in the portable engine power tool, it is not only required to efficiently perform the forced cooling in the engine operation but also required to promote the natural cooling for the cylinder and the insulator so as to reduce the increase in the temperature of the carburetor in order to suppress the increase in the temperature of the carburetor without making the insulator long, and the requirement is an important technical problem.
Japanese Utility-Model Application Laid-Open Publication No. H06-18622 (Patent Document 1) describes an engine in which the cooling air passing through a spark plug is regulated by suspending an air guiding rib toward the cylinder on an upstream side of the spark plug so as to form a small space between the cylinder and the air guiding rib.